1. Field of the Invention
This invention relates to a method for measuring the time-of-flight (TOF) of ultrasonic waves through materials, and specifically a method of identifying specific cycles in a received signal packet. The ability to identify a specific cycle eliminates a source of ambiguity in measuring the TOF of ultrasonic waves, thereby substantially improving the robustness and accuracy of systems which rely on TOF measurements.
2. Description of Related Art
There are many methods and devices which use ultrasonic waves to measure the tensile load on a load bearing member (such as a fastener).
U.S. Pat. No. 4,294,122 (to Couchman) discloses a fastener having an acoustic transducer built into its head or threaded end, and a method using the pulse echo technique to measure the pre-load stress. The method includes measuring the time for two sets of echoes to travel the length of the fastener, one set prior to pre-load and the other set during torquing of the fastener. Then, by knowing the material constant M, the grip length .delta., the diameter D, the parameter for correcting the stress distribution .alpha., and the travel time of the echoes, the stress S can be measured to obtain an accurate measure of bolt pre-load by using the following formula: EQU S=(M.vertline.(.delta.+.alpha.D)).times..DELTA.T
Another patent disclosing the pulse echo time measurement technique is U.S. Pat. No. 4,471,657 (to Voris et al.). The '657 patent discloses an apparatus and method for measuring the length and stress in a load bearing member such as a fastener. The method includes measuring the time it takes two signals having the same frequency but a pre-determined phase difference to travel the length of a load-bearing member; detecting the longer of those travel times; compensating for the phase difference; and using an intelligent processing and control means to receive the time interval data and process the data to produce an accurate measure of the change in fastener length or the stress applied thereto. The apparatus includes an ultrasonic transducer permanently or temporarily in contact with the fastener.
U.S. Pat. No. 3,918,294 (to Makino et al.) describes a method of measuring axial strains applied to a bolt. An ultrasonic wave is applied to a bolt to generate forced oscillations therein and two different natural frequencies are measured in the bolt, one of which is measured when the bolt is under little or no axial force, the second of which is measured when the bolt is under axial strain. The ratio of change or the differential between the first and second frequencies is obtained and is compared to calibration data for the axial strain verses the ratio of change or differential.
Also, U.S. Pat. No. 4,569,229 (to de Halleux) teaches a method for measuring strains in load-bearing members which eliminates the need for calibration for grip length. The method comprises measuring the time an echo travels from the top of a load-bearing member to an artificial reflector and back. The artificial reflector includes vertical and horizontal boar holes or perforations in the load-bearing member. The transit time of the wave in the bolt is dependent on the stress the bolt is under.
Other stress measurement methods and devices allow the user to measure the change in stress during tightening of the load-bearing member. For example U.S. Pat. No. 4,363,242 (to Heyman) discloses using a pulsed phased-locked loop technique to measure changes in strain in a load-bearing member. The phase of a radio frequency wave is compared to the phase of a wave supplied by a continuously running voltage controlled oscillator. Then, when the load bearing member is under stress, the tension and sound velocity (which are dependent on the strain) cause an acoustic phase shift which produces a frequency shift (.DELTA.F) in the voltage controlled oscillator. The frequency shift divided by the frequency (F) is linearly proportional to the applied load. Heyman '242 displays frequency changes which are indicative of changes in the load on the bearing member. This technique requires that the ultrasonic sensor be kept on the load-bearing member during tightening and, thus, load measurement of a previously tightened load bearing member is not possible.
In contrast, several references describe methods of measuring the load on a load-bearing member which is already under tension. For example, U.S. Pat. No. 5,237,516 (to Heyman) describes a method of recertifying a load on a bearing member using a pulsed-phase, locked-loop system. The method includes comparing the phase of an ultrasonic tone burst applied to the load-bearing member (via a transducer) to the phase of a tone burst reflected through the bearing member, and adjusting a sample/hold for selecting a phase point of the reflected tone burst. The pulsed phase-locked loop system can be locked such that the phase is constant and the output frequency of the voltage controlled oscillator indicates the load applied to the bearing member. In this way the stress on a tightened bolt can be determined.
Similarly, Froggatt et al., "Interrupted Ultrasonic Bolt Load Measurements Using the Pulsed Phase Locked Loop System," IEEE Trans. on Instrumentation and Measurement, Volume 45, No. 1, February 1996, pp. 112-16, describes a method of acquiring a previous phased lock point using a pulsed phase-lock loop ultrasonic system. This method focuses on analyzing the pulsed phase locked loop in the time domain rather than in the conventional frequency domain. A systematical procedure for making the measurements is described which is not dependent on the qualitative judgment of the test operator.
In addition, several references have described using time-of-flight measurements of longitudinal and shear waves to calculate tensile stress in load-bearing members. For example, Bobrenko et al., "Ultrasonic Method of Measuring Stresses in Parts of Threaded Joints," All Union Scientific Research Institute of Non-Destructive Testing, Kishinev, Translated from Defektoskpiya, No. 1, pp. 72-81, January-February 1974, and Johnson et al., "An Ultrasonic Method for Determining Axial Stress in Bolts," A Journal of Testing and Evaluation, Volume 14, No. 5, pp. 253-59, September 1986, describe methods for determining stresses in load bearing members by measuring the time-of-flight required for longitudinal and shear ultrasonic waves to travel up and down the length of the load-bearing members. In Bobrenko et al. and Johnson et al. the user is required to know the length of the load-bearing member in order to make a stress measurement. In both references, the stress on the bolt can be measured where only one end of the bolt is accessible.
Also, U.S. Pat. No. 4,602,511 (to Holt) teaches a method using the times of flight of both longitudinal and transverse waves to determine the stress in a load-bearing member. Both Holt and Johnson et al. do not require a stress measurement to be taken when the load bearing member is under zero stress.
As the above-discussed references indicate, the prior art is replete with references which disclose the use of piezoelectric materials embedded in or attached to load-bearing members to measure the stress in the load bearing member. Additional examples include U.S. Pat. Nos. 4,846,001 and 5,131,276 (to Kibblewhite) which describe the use of piezoelectric elements and polymers permanently attached to load-bearing members with adhesives or through a vapor deposition technique. These transducers are compatible with the above-described pulse-echo techniques used for load measurement and have the additional advantages of not having coupling induced errors, and they facilitate generation of transverse waves.
Other references of interest include G. C. Johnson, "On the Applicability of Acoustoelasticity for Residual Stress Determination," Journal of Applied Mechanics, Volume 48, No. 4, 1981, pp. 791-795; and J. S. Heyman and E. J. Chem, "Ultrasonic Measurement of Axial Stress," Journal of Testing and Evaluation, Volume 10, No. 5, pp. 202-211, September, 1992.
The above-discussed prior art cannot accurately measure the time-of-flight of ultrasonic waves in load bearing members. The measurement errors have magnitudes which are multiples of the period of the carrier frequency of the associated signal bursts. Ambiguity in identifying corresponding cycles between two received echo signals is the cause of this error. Errors in load determination may result particularly when the measured load changes suddenly (as when tightening with impact and impulse tools) or when techniques such as described above in Holt are used to make absolute measurements of load.
The pulse-echo-overlap method has been used to measure ultrasonic time-delay and to accurately measure the cyclic overlap and phase velocity. For example, Papadakis, Emmanuel P., Ultrasonic Velocity and Attenuation: Measurement Methods with Scientific and Industrial Applications, Volume 12, pp. 277-97 (Edited by Mason, Warren P., 1976) and Ultrasonic Phase Velocity by the Pulse-Echo-Overlap Method Incorporating Diffraction Phase Corrections, J. Acoust. Soc., 10.6; 11.3, pp. 1045-51 (1967) describe a method of measuring ultrasonic wave velocity and travel time in materials and structures. The method measures the TOF of radio frequency signal bursts in nondispersive media using an ultrasonic time intervalometer, an oscilloscope, and a transducer on a buffer rod. The correct determination is dependent on the McSkimin .DELTA.t criterion which is defined as: ##EQU1##
where .function..sub.L and .function..sub.H (.function..sub.H is the resonant frequency (.function..sub.R) of the transducer) are the higher and lower frequencies differing by about 10%; P is the number of round trips between the echoes used in the measurement; .gamma..sub.L the phase shift characteristic of the specimen-transducer interface at the low frequency; .gamma..sub.H is the phase shift characteristic of the specimen-transducer interface at the high frequency; n is the number of cycles of mismatch from echo to echo; and t.sub.L and t.sub.H are the TOF values for the two frequencies. For example, .function..sub.L may be about 0.9 .function..sub.H.
To overcome the shortcomings of not providing reliable and robust time-of-flight measurements of ultrasonic waves in materials, a method of measuring the time-of-flight of ultrasonic waves in materials is provided. An object of the present invention is to provide a method of making accurate and reliable ultrasonic tensile load measurements, for example, with impulse and impact fastener assembly tools.